1. Field of the Invention
The present invention relates to a method for providing a secondary (rechargeable), lithium-ion electrochemical cell with graphitic carbonaceous anodes having improved properties. Specifically, the invention relates to a method for reducing and/or preventing the exfoliation of the graphitic carbonaceous electrode of a lithium-ion cell, wherein the exfoliation is caused by the intercalation of electrolyte solvent along with lithium ion into the graphitic carbonaceous electrode. This method is accomplished by adding one or more chelating polyamines to the electrolyte solution of the lithium-ion cell. The novel method and the improved lithium-ion cell are claimed herein.
2. Prior Art
Batteries in the prior art generally contain a metal anode, a cathode and an electrolyte which incorporates ions of the metal employed as the anode. When the battery is discharged, metal ions originating from the anode enter the electrolyte and travel to the cathode to generate electrical energy. Assuming that the reaction between the metal ions and the cathode can be reversed, this can be accomplished by applying electrical energy to the battery cell, therefore providing a recharged cell. This type of cell is referred to in the art as a secondary cell.
Secondary cells have been made using alkali metals as the anode material. It is well known that alkali metals are inherently unstable in water and react violently in the presence of water. Because of this, care must be taken to build these types of batteries without exposure of the cell to any ambient moisture or any other source of water. Moreover, the prior art further recognizes that the use of, for example, lithium metal electrodes creates an adverse effect upon the cycling performance and the safety in the finished secondary cell. Replacing alkali metal anodes with a lithium intercalation host, such as graphitic carbonaceous materials, has been found to alleviate the concerns resulting from the use of the metal anodes. Graphitic carbon provides the flattest discharge profile and operation closest to the lithium potential.
Batteries employing a lithium intercalation host as the anode and a lithium salt electrolyte are, by definition, called "lithium ion batteries." The present invention relates to these types of batteries.
Although the use of graphitic carbonaceous electrodes alleviates the issues relating to the use of alkali metal electrodes in secondary cells, other problems are incurred by their use. The problems incurred with use of graphite electrode batteries results from the cointercalation of lithium ion and electrolyte solvent molecules into the graphite anode upon initial charging of the cell. The cointercalation of lithium ion and electrolyte solvent molecules into the graphite anode causes exfoliation of the graphite electrode, irreversible loss of cell capacity and electrolyte solvent, and an overall reduction in cell performance. The issues associated with the use of graphitic carbonaceous materials as anodes in lithium-ion cells, along with theories associated with these issues, are described in detail in Wilkinson et al., U.S. Pat. No. 5,130,211, the disclosure of which is incorporated herein by reference.
In addition to describing the issues associated with the use of graphitic carbonaceous materials as anodes in lithium-ion cells, Wilkinson et al. provides a means to address these issues. Wilkinson et al. teach that the addition of sequestering agents, such as crown ethers and glymes, as well as cryptands and other macroheterocyclic compounds (column 6, lines 5+), to the electrolyte solution of lithium-ion cells prevents/suppresses exfoliation of the graphite electrodes therein upon the initial intercalation of an alkali metal in the cells. The large crown ethers or tetraglyme molecules, for instance, tightly chelate the lithium ions and prevent the solvent molecules from passing into the graphite layers along with the lithium ions. Unfortunately, crown ethers are very expensive, and in all likelihood too costly for most commercial and military applications.
The prior art further teaches that chelating agents may and have been employed as additives to electrolyte compositions in various battery cells and for various purposes. It is important to note, however, that aside from the teaching of Wilkinson et al., the prior art does not suggest the addition of chelating agents to electrolyte compositions for the purpose of preventing exfoliation of graphite electrodes in lithium ion cells.
Langer et al., U.S. Pat. No. 3,764,385, teach the use of monomeric or polymeric, polyfunctional chelating tertiary amines as chelating agents that could be used as electrolyte additives in an electric battery cell so as to form a complex of an inorganic lithium salt. The chelating agents taught could complex with lithium ion so that lithium salts with lattice energy as high as 210 kilocalories per mole could be dissolved in nonpolar solvents such as toluene to concentrations exceeding 1M. Among the many chelating agents taught are N, N, N', N", N"-pentamethyldiethylenetriamine (PMDT), N, N, N", N', N"', N'"-hexamethyltriethylenetetramine (HMTT) and N, N, N', N'-tetramethylethylenediamine (TMED). The battery cell taught by Langer et al., however, is not a lithium-ion cell (the electrodes may be composed of lithium). Moreover, the reference does not address the exfoliation issue of interest herein. No suggestion is made by this teaching to employ these chelating agents in a lithium-ion cell.
A non-aqueous electrochemical cell wherein the electrolyte is comprised of a salt complexed by a tertiary amine and an aromatic organic solvent is taught by Whitney and Foster, U.S. Pat. No. 4,670,363. Whitney and Foster found that the conductivity of these types of electrolyte solutions could be raised over an order of magnitude with the addition of 10% or more of a polar solvent having a dielectric constant greater than 20, such as propylene carbonate or sulfolane. The addition of the tertiary amines act as conductivity enhancers in organic lithium electrolyte solutions. The battery taught by Whitney and Foster is not a lithium-ion battery.
Use of polyamines as electrolyte additives in electrochemical cells are not well known in the electrochemical community; and their use in lithium-ion batteries, in particular, has not been explored.
Although the use of graphitic carbonaceous electrodes in lithium-ion batteries has been taught in the prior art, and the problem of exfoliation identified, the need still exists to provide a rechargeable lithium-ion battery that solves the exfoliation problem in a simple and cost effective fashion. The present invention addresses this issue by providing a method and means for preventing the exfoliation of the graphitic carbonaceous anode by adding readily available chelating agents to the electrolyte so as to prevent the cointercalation of the electrolyte solvent into the graphite electrode during the cell discharge.
Moreover, the addition of the chelating agents within the scope of the present invention to the electrolyte of the lithium-ion cell will provide an electrolyte which allows reversible intercalation of lithium ions with less exfoliation of the graphite anode or excessive irreversible capacity loss. In general, the present invention provides a method for improving the overall cell performance of lithium-ion electrochemical cells having graphitic carbonaceous electrodes therein.